The present invention relates to a variable power assist steering system and more particularly to a variable power assist steering system employing an operating noise free fluid flow distributor circuit.
EP No. 0 041 887 B1 (corresponding to JP No. 57-30663 A1) discloses a fluid flow distributor circuit for a power assist steering system. This known fluid flow distributor circuit includes two parallel branch fluid paths, each being provided with two kinds of variable flow orifices connected in series. One kind of these variable flow orifices varies its flow area in inverse proportion to a steering torque during clockwise turning of a steering wheel, while the other kind varies its flow area in inverse proportion to steering torque during counterclockwise rotation of the steering wheel. A power cylinder has cylinder chambers connected to the two fluid paths at cylinder connection ports disposed between the two kinds of variable flow orifices. In order to prevent formation of cavitation in liquid flow, a bypass circuit including an auxiliary variable flow orifice and a fixed orifice connected in series is arranged in parallel to the main variable flow orifices disposed downstream of the cylinder connection port of each of the branchfluid paths. The auxiliary variable flow orifice is designed such that it varies its flow area in inverse proportion to steering torque during the same rotation of the steering wheel as the rotation of the steering wheel during which the associated main variable flow orifice varies its flow area in inverse proportion, but the former decreases its flow area to the mimimum after the latter has decreased its orifice area to the minimum as steering torque increases. This circuit arrangement is found to be effective in preventing noise resulting from forming of cavitation in hydraulic fluid flow passing through the fluid flow distributor circuit.
However, with this circuit arrangement, it is not possible to vary power assistance with variation of vehicle speed. Since the degree of power assistance is determined by characteristics of the auxiliary variable flow orifice valve that is designed to be closed at a relatively large steering torque, it is not posible to produce a sufficient degree of power assistance.
An object of the present invention is to provide a power steering system which, without any operating noise, has power assitance change with vehicle speed or any other variable.